It is known that similar parts will produce similar patterns of vibration amplitudes over a range of frequencies with the greatest amplitudes being at the resonant frequencies. When one of these parts has one or more defects, a substantial variation in the pattern will be produced and it is known to use this variation to detect flaws in production parts.
The above-known principles were employed in U.S. Pat. No. 3,043,132 which is incorporated herein by reference. In the case of this patent, the resonant frequencies of a part are first obtained and then used to determine circuit values in order to provide for output signals indicating whether or not a part being tested has resonances corresponding to a standard. In accordance with this invention, all such preliminary computations are eliminated by simultaneously comparing the part being tested with a standard part known to be free of defects or with another portion of the part being tested expected to have the same amplitude pattern as the portion being tested.